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Self-sustaining vibration and mechanical resonance effects in stimuli responsive liquid crystal polymer coatings and membranes (Vibrate)

Periodic Reporting for period 3 - VIBRATE (Self-sustaining vibration and mechanical resonance effects in stimuli responsive liquid crystal polymer coatings and membranes (Vibrate))

Reporting period: 2018-10-01 to 2020-03-31

The objective of the project is to bring the molecular action, in conjunction with the macroscopic deformation, out of its equilibrium aiming a self-sustaining oscillation of the macroscopic response to a continuous trigger. An example that will be investigated is a surface that continuously changes it topography when addressed by its trigger. Another example is a membrane that will oscillate its (localized) free volume thus providing an active transport mechanism for species through the membrane. Alternatively we will investigate the response to an oscillating trigger with a frequency matching the molecular action to find sweet spots for mechanical resonance thus enhancing the macroscopic effect. The research is challenging but when achieved it will undoubtfully lead to new applications in coating and film technology with an outlook to soft robotics, self-pumping membranes, mechanical communication at man-machine interfaces and energy harvesting.
In the Vibrate project the emphasis will be on light and electricity. Three tasks were defined:
• Task a. The generation of responsive materials and processing them into devices
• Task b. Studies of the device responses to light and/or electrical fields
• Task c. Realization of self-sustained oscillation and resonance effects

We worked simultaneously on these tasks with our PhD and Postdoctoral students.

Task a
Photo-actuation of a liquid crystal network (LCN) or elastomer (LCE) is normally established by covalently incorporation of azobenzene molecules into the network. Besides the feature of photo-deformation, these molecules have disadvantages such as slow back relaxation which is a problem in the creation of vibrational motion. For that, we developed a set of reactive azobenzene molecules, that have a fast back relaxation reaction (Nature 2017). We also developed a set of new fast trigger molecules that have their absorption predominantly in the UV part of the spectrum and are therefore colorless (Advanced Materials 2017). Some of these molecules form the basis of photo-stabilizers in commercial plastics which provides the prospect of better durability during light-induced actuation. With these photo-triggers we made a set of oscillating devices (Advanced Materials 2017) and a mill-like device of which the blades rotate under light exposure (Tetrahedron 2017) and perpetual morphing devices capable to create continuous waves (Nature 2017). To expand on the flexibility on the choice of actuation wavelength, we introduced a set of pH sensitive azo dyes. Depending on the pH history the absorption can be positioned at a desired location, bringing origami type of morphing a step closer to realization (Angewandte Chemie 2017). In addition, we developed a method to produce an array of fibers attached to a substrate. Upon actuation the fibers are capable to transport species (Advanced Functional Materials 2016).

Task b
We focus on coatings adhering to a solid substrate that dynamically change their surface. One approach is by addressing the local scalar order parameter. At the contact line between orthogonal directors (Soft Matter 2017) or at the position of topographical defects (Nature Communications 2018) surface elevations or surface indentations are formed. Modulating the polarization of UV light the surface deformation was made dynamic (Crystals 2017). A second mechanism is by utilizing generation of free volume by double wavelength excitation of azobenzene crosslinker molecules (Nature Communications 2015). By dynamic mechanical analyses under exposure we could demonstrate that the oscillating dynamics of the azobenzene leads to photo-softening (Soft Matter 2016). Electricity was also used to initiate surface motility. A chiral-nematic LCN is applied on a glass substrate provided with an interdigitated electrode structure. The helix axes were aligned parallel to the substrate. This induces a fingerprint texture with alternating elevations and indentations. We studied the formation mechanism of the structures and could control them (ACS Applied Materials & Interfaces 2020). When actuated with an AC field, the surface topography inverts (Advanced Materials 2017). We developed membrane technologies. We developed a photo-sponge which is a thin LCN polymer coating that on command absorb and eject a liquid using light as the trigger (Advanced Functional Materials 2018) and RF-AC field (Matter 2020). Related to this, we developed an elevator principle to transport material (gases, liquids). We demonstrated the switching of nitrogen flux through the membrane by either light or temperature (Soft Matter 2019, Advanced Functional materials 2019).

Task c
A first example of a self-sustained oscillation is the continuous wave in an UV exposed film as discussed under task a and published in Nature 2017. Of even more interest for potential application are the oscillatory and resonance effects in coatings observed in electrically generated surface topographies (Nature Communications 2017). An important observation, which is in the heart of the objectives of our Vibrate project, is that the resonance effects are instable, i.e. as soon as the topographies form the resonance conditions change. This provides a feedback loop because as soon as the surface has relaxed to its initial state, the resonance conditions are met again and surface deformation is re-initiated. In collaboration with Wageningen University, we used frequency-resolved laser speckle imaging which resulted in two publications (Nature Communications 2019, ACS Applied Materials & Interfaces 2020).
There are a number of project results within Vibrate that are unprecedented and beyond the state of the art of existing materials:

1. We developed organic coating materials that show topographical deformation when addressed by an in-plane AC field. Electrodes are 'buried' in the coating to avoid touch. Topographies form when the field frequency is above a threshold, meeting resonance conditions. Unexpectedly, on top of the primary deformations there is a high frequency smaller-sized oscillation. In the third period of the project we generated more insight in their formation and resonance mechanisms by laser speckle measurement and Fourier analysis.
2. We created coatings with a fingerprint surface structure with controlled hills and valleys of the order of 100 nm. By an electrical field the surface topography inverts, capable to transport material.
3. Vibration and continuous motility was the main objective of the project. We developed a number of examples: vibrating surfaces, oscillating polymer beams, a polymer mill with light-driven blades, plastic films with a continuous wave-like deformation. When the wave-forming plastic film is mounted in a frame, it walks over the surface away from the light source or towards the light source, depending on molecular alignment.
4. We developed self-pumping membranes capable to transport or to secrete liquids or gases, either by light or RF electricity.
Removal of sand from a surface with a vibrating topography
A liquid crystal polymer film that deforms in a continuous wave when exposed with light.